Low Voltage Differential Signaling (LVDS) and Current Mode Logic (CML) are two standards commonly used for differential signal transmission. These standards are especially common in the field of high-speed serial (HSS) interfaces. Each standard has its own advantages and disadvantages, making it better suited for either alternating current (AC) coupling or direct current (DC) coupling.
The performance of an LVDS or CML transmitter is characterized by several attributes, including its common-mode voltage, maximum achievable voltage swing, and power supply requirement. Common-mode voltage is the bias point about which the transmitter's output voltage varies, and is defined as the average of the high signal voltage (Vhigh) and the low signal voltage (Vlow). Maximum achievable voltage swing is the greatest amount by which Vhigh can differ from Vlow while keeping all transmitter devices in saturation. Finally, power supply requirement is the minimum power supply voltage (Vdd) that can be used with the transmitter, and is often expressed in terms of the voltage swing. Other considerations, such as the area occupied by the transmitter circuit, are discussed later.
LVDS transmitters tend to be more well-suited to AC coupling than CML transmitters. Under AC coupling conditions, an LVDS transmitter will often permit a higher voltage swing and a lower power supply voltage than a CML transmitter. Although LVDS transmitters tend to have a lower common-mode voltage under these conditions, a high common-mode voltage is not essential to AC coupling.
In contrast, CML transmitters tend to be more well-suited to DC coupling than LVDS transmitters. Under DC coupling conditions, a CML transmitter will usually allow a higher voltage swing, a lower power supply voltage, and a higher common-mode voltage than an LVDS transmitter.
In view of the foregoing, it would be desirable to combine the benefits of LVDS AC transmission with the benefits of CML DC transmission in a single transmitter circuit. Furthermore, it would be desirable to make switching between the two transmission modes relatively simple.